Optical Remote Sensing

Optical remote sensing makes use of visible, near infrared (NIR) and short-wave

infrared (SWIR) sensors to form images of the earth's surface by detecting the

solar radiation reflected from targets on the ground. Different materials reflect

and absorb differently at different wavelengths. Thus, the targets can be

differentiated by their spectral reflectance signatures in the remotely sensed

images. Optical remote sensing systems are classified into the following types,

depending on the number of spectral bands used in the imaging process.

• Panchromatic imaging system: The sensor is a single channel detector

sensitive to radiation within a broad wavelength range. If the wavelength

range coincide with the visible range, then the resulting image resembles a

"black-and-white" photograph taken from space. The physical quantity

being measured is the apparent brightness of the targets. The spectral

information or "colour" of the targets is lost. Examples of panchromatic

imaging systems are:

o IKONOS PAN

o SPOT , HRV-PAN

• Multispectral imaging system: The sensor is a multichannel detector with

a few spectral bands. Each channel is sensitive to radiation within a narrow

wavelength band. The resulting image is a multilayer image which contains

both the brightness and spectral (colour) information of the targets being

observed. Examples of multispectral systems are:

o LANDSAT MSS

o LANDSAT TM

o SPOT , HRV-XS

o IKONOS MS

• Superspectral Imaging Systems: A superspectral imaging sensor has many

more spectral channels (typically >10) than a multispectral sensor. The

bands have narrower bandwidths, enabling the finer spectral

characteristics of the targets to be captured by the sensor. Examples of

superspectral systems are:

o MODIS

o MERIS

• Hyperspectral Imaging Systems: A hyperspectral imaging system is also

known as an "imaging spectrometer". it acquires images in about a

hundred or more contiguous spectral bands. The precise spectral

information contained in a hyperspectral image enables better

characterisation and identification of targets. Hyperspectral images have

potential applications in such fields as precision agriculture (e.g.

monitoring the types, health, moisture status and maturity of crops),

coastal management (e.g. monitoring of phytoplanktons, pollution,

bathymetry changes). An example of a hyperspectral system is:

o Hyperion on EO1 satellite